The “multi-hit” requirements for transparent armor (“TA”) are driven by need in the conflicts. In today's military missions the encounter with the adversary is at closer distances than in most prior, “conventional” warfare situations, and there is a greater occurrence of sniper situations. Consequently, in consideration of the present situations, the multi-hit specifications for TA typically require the distance between the shots received by the TA be much smaller than the 100-120 mm distance used in older specifications. One specification that is currently in wide use calls a “T-pattern” in which a shot spacing of 50 mm is required along the two axes of the “T.” To be within the allowable tolerances of the specification, the shot spacing can be as small 20 mm during testing and still qualify as “valid.” For a glass-based TA, meeting the new T-pattern specifications is an industry-wide challenge.
In addition to multi-hit requirements, it is required that new TA systems or laminates be of lower weight that the prior systems or laminates. As a result, new materials are starting to play a role in managing TA weight. These new materials include transparent ceramics of spinel, ALON and sapphire, transparent glass-ceramics, and other high performance glasses such as borosilicate glasses. However, due to cost/performance tradeoffs, these materials are typically used as a strike-face material to effectively break up or deform a hard projectile; and glass and/or polymer layers are placed behind the strike-face to form the remainder of a TA laminate and defeat the projectile.
To meet the tight multi-hit requirement, it is imperative that the strike-face material remain part of the TA's functionality for all shots, rather than for just the first impact. The key to multi-hit capability is to retain the strike-face integrity and, ideally, to also reduce the size of damage during impact, thus enabling effective defeat of multiple impacts. A glass cover sheet or layer over the strike-face is a natural choice. A plastic-based cover is lightweight, and can be effective in retaining material, but it lacks the scratch resistance needed for an outer-most layer. Conventional wisdom appears to have driven manufacturers to use a 6 mm or 10 mm glass layer as a cover to provide the perceived need for scratch and rock-strike resistance. However, the use of such 6 mm or 10 mm cover glasses added weight to the TA laminate and it has also been found to greatly reduce the strike-face performance, rendering the whole multi-layer laminate less effective in performance. A possible explanation is that the weak cover glass can be the initiation point of a fast traveling failure wave which pre-conditions and weakens the strike-face ahead of the projectile. This is undesirable. Consequently, there remains a need for an effective cover that improves multi-hit performance without compromising scratch and rock-strike properties.